It is known in the art that unwanted electromagnetic radiation can be absorbed by organic dyes or by inorganic colorants impregnated in a substrate material. U.S. Pat. Nos. 4,935,166 and 4,106,857 disclose absorption filters utilizing organic dyes and inorganic colorants, respectively. The principal disadvantage of the dye approach is that peaks of maximum absorption cannot be tuned to coincide exactly with emission wavelengths of the unwanted monochromatic radiation. The spectral distributions of absorbing visible colorants are generally slowly varying with wavelength and tend to lack the sharp cuton and cutoff profiles that are essential for obtaining high photopic efficiency. Absorption filters utilizing organic dyes cannot be design engineered to operate at any selected wavelength to provide a high degree of optical density, and they have not generally provided narrow rejection bands for unwanted radiation sources. As a result of these factors, the visual photopic transmittance response of dye based laser rejection filters must be sacrificed in order to achieve high optical densities required to absorb unwanted radiation and provide eye protection. It should also be noted that the radiation absorbed by a dye material is usually dissipated in the form of heat or fluorescence or by photochemical reactions within the material. Under some circumstances of exposure to high energy radiation, this can lead to damage of the dye material which no longer shields the user from the potentially hazardous optical radiation sources.
Rugate filters are disclosed in U.S. Pat. Nos. 4,837,044, and 4,915,476. These filters use graded (inhomogeneous) multilayers of dielectric thin film materials, typically mixtures of inorganic silicon dioxidesilicon nitride (SiO.sub.2 --Si.sub.3 N.sub.4), mixed zinc sulfide-selenide, or aluminum arsenide/gallium arsenide on an optically transparent support. However, these types of filters have proven difficult to manufacture outside of the laboratory.
Certain minus filters are known in the art. U.S. Pat. No. 4,896,928 discloses multilayers of homogeneous inorganic oxides in thin film structures on an optically transparent or partially absorbing support. Typically such filters are not efficient in providing ultra narrow reflection bandwidths for the rejection of unwanted electromagnetic radiation.
Plasma polymerization of certain polymeric materials including hexamethyl disiloxane, structure, properties, and variation of refractive indices with deposition conductions are disclosed in Herman V. Boeing, "Fundamentals of Plasma Chemistry and Technology", Technomic Publishing Co., Inc., Lancaster, Pa., 327-332 (1988); J. J. Beulens et al., Journal of Applied Polymer Science:Applied Polymer Symposium 46 91-108, 209-242, 527-540 (1990).